Voice communication between people or with one person located in an environment having significant background or ambient noise can be difficult, and burdensome--in some cases even dangerous if miscommunication occurs. Additionally, working in such an environment can become impossible in some circumstances if there is no shielding of the worker from the noise. As a result of this need to communicate or to shield workers from excessive ambient noise, various devices have been developed.
It has been stated that a typical approach for noise attenuation or shielding in the workplace is to provide workers with headsets having high mass, large internal volume and a spring support that exerts heavy pressure upon the head, i.e. forces the earcup against the head of the wearer. The high mass and heavy pressure operate to create a seal with the wearer's head which in turn serves to attenuate low frequency noise while the large internal volume provides so-called high frequency roll off. The problem with such a passive noise attenuation approach was said to be the discomfort associated with the wearing of such devices.
Other previously described headsets utilized an active noise attenuation approach, wherein a cancellation or anti-noise signal is generated and added to the signal being applied to the headset. Upon being acoustically reproduced, the anti-noise component tends to cancel the background or ambient noise within the region around the ear. Such prior active attenuation techniques are generally of two types, so-called open loop headset systems and closed loop headset systems.
An example of a headset system incorporating open loop components can be found in U.K. Patent Application No. 2,104,754 A - Chaplin, published Mar. 9, 1983, wherein a sensor is used to detect a repetitive noise rate or frequency, which in turn is used by a waveform generator to generate a cancellation signal. The closed loop components depicted in this U.K. application will be discussed more fully below. The problems associated with open loop headset systems are first, they may be limited to only cancelling certain background noise and second, it is difficult to take passive headset attenuation into account. Closed loop headset systems, however, do account for passive attenuation and tend to cancel all background noise.
An early closed loop headset system can be found in Olson, H., Acoustical Engineering, Van Nostrand, New York 1957, pps. 415-418. Subsection C, Headphone-Type Noise Reducer, describes a headset wherein a microphone has been placed in an earcup closely adjacent to a diaphragm for the purpose of providing noise reduction. The microphone senses the pressure in the cavity formed by the earcup on the ear and provides a feedback signal representative of such pressure. It is understood that the pressure in the cavity is reflective of not only the acoustic reproductions of the driver but also external noise which has penetrated the earcup. The microphone signal can be used to cancel or null the external noise by shifting the phase of the signal. Consequently, when acoustically reproduced, the noise component of the microphone signal tends to acoustically cancel external noise present in the cavity.
Another closed loop system can be found in the report AD-A009 274 entitled A STUDY OF PROPOSED EAR PROTECTION DEVICES FOR LOW FREQUENCY NOISE ATTENUATION by P. M. Dallosta dated April, 1975 at pages 86, 87 and 117-119. The report depicts a microphone within an earcup for use in a cancellation circuit. The cancellation circuit is shown at pages 117-119 to sum the communications signal with the microphone signal after the microphone signal has been provided proper gain and phase shift for optimum cancellation. The resultant summed signal is amplified and supplied to the headset.
As indicated above, U.K. Patent Application No. 2,104,745 also discloses closed loop components. Specifically, a microphone is said to be provided closely adjacent a speaker in a headset earphone. The signal from the microphone is fed back to an "adaptive means" which in turn utilizes the microphone signal and the previously described open loop signal to generate a cancellation signal. When the cancellation signal is reproduced by the speaker, it is said that the noise field is nulled except for certain desired sounds. It is also indicated that the microphone signal could be used in a direct feed-back system for attenuating mid-band frequencies.
U.S. Pat. No. 4,455,675 - Bose et al. discloses a closed loop headset system wherein a microphone is mounted coaxially with a driver in a headphone. The open or vented region of the microphone is directed away from the driver and towards the ear canal. The signal generated by the microphone is combined with the signal desired to be reproduced by the driver. Prior to providing the combined signal to the driver, the combined signal is said to be processed by passing it through a compressor to limit the level of high level signals and thereafter it is applied to a compensator to ensure that the open loop gain meets the so-called Nyquist stability criteria to prevent system oscillation.
Somewhat related U.S. Pat. No. 4,644,581 - Sapiejewski discloses a closed loop headset system similar in design to that shown in U.S. Pat. No. 4,455,675 except for two (2) features. Damping material has been positioned to cover the headphone cavity which contains the driver. Also, instead of orienting the microphone coaxially with the driver and directing the open or vented microphone face towards the ear canal, the microphone is located off-set from the driver axis and oriented so that its diaphragm is perpendicular to a plane containing the driver diaphragm, i.e. the open or vented face is directed perpendicular to the driver axis. The perpendicular orientation is said to result in increased bandwidth of the closed loop and the off-set location is said to reduce peaks in frequency response at the high end.
The problem which remains despite these prior open and closed loop headset systems, is that phase lag, particularly at high frequencies, has not been minimized. Such phase lag can occur as a result of several factors, one of which is the propagation delay associated with the distance between the microphone and driver.